Dementia in children or young adults is most frequently caused by neuronal ceroidlipofuscinoses (NCL), a group of incurable lysosomal storage disorders linked by the accumulation of a characteristic intracellular storage material and progressive clinical deterioration, usually in combination with visual loss, epilepsy, and motor decline. The clinical characteristics can vary and the age at disease onset ranges from birth to over 30 years. Diagnosis of an NCL is difficult because of genetic heterogeneity with14 NCL forms (CLN1-CLN14) identified and a high phenotype variability. A new classification of the disorders is based on the affected gene and the age at disease onset and allows a precise and practicable delineation of every NCL disease. We present a clear diagnostic algorithm to identify each NCL form. A precise diagnosis is essential for genetic counseling of affected families and for optimizing palliative care. As patient management profits from recognizing characteristic complications, care supported by a specialized team of NCL clinicians is recommended. The development of curative therapies remains difficult as the underlying pathophysiological mechanism remains unclear for all NCL forms.